1. Field of the Invention
The invention relates to a screen changer for use in a system that processes flowable plastic. More particularly, the invention relates to a screen changer that has a movable element that is moved between two positions by a hydraulic cylinder.
2. Description of the Prior Art
Screen changers for use in production systems that process flowable plastic are known in the industry. They use a so-called storage hydraulic and, thus, have a reservoir for hydraulic fluid that can be pressurized. This enables the use of high-pressure pumps that are able to build up a relatively high pressure at relatively low capacity per unit of time. A high pressure is therefore built up over a relatively longer period of time in the hydraulic reservoir, so that a large amount of hydraulic fluid with correspondingly high pressure is made available to the screen changer.
A portafilter can thus be moved or shunted aside a long distance within a short period of time by means of the large amount of fluid, when the screen is to be changed. If the portafilter has two filters, this allows a dirty screen to be moved out of the material feed channel, and, at the same time, a clean screen that provides a high degree of permeability, to be pulled into the material channel. The dirty screen, in this out of the way position, is in the change position, where it is accessible and can be cleaned or, if necessary, exchanged for an unused or previously cleaned screen, while production continues with the clean screen.
The mobility of the portafilter can, however, be hampered, for example, by the adherence of baked-on plastic residues. In such cases, it is necessary to overcome a high resistance to movement, in order to move the portafilter. For this reason the fluid amount stored in the hydraulic reservoir is under the aforementioned high pressure, which makes it possible to overcome such resistance.
It is also necessary to be able to shunt aside a movable component as quickly as possible when operating bypass, start-up, and switching valves in systems that process molten plastic, similar to the shunting movement with portafilters. In these cases, too, however, it may also be necessary to overcome a high resistance, such as an impedance to movement caused by the adhesion of baked-on plastic residues. In general, therefore, components such as portafilters, bypass valves, start-up valves, and switching valves are hereinafter referred to in each case as a “movable element”.
It is a disadvantage with conventional screen changers, that the storage hydraulic requires a relatively expensive hydraulic reservoir, which, due to the pressure it must withstand, is relatively expensive to produce and particularly, also requires routine technical inspections. These inspections incur significant follow-up costs and must be performed at regular intervals.
The task of the invention is to improve a generic screen changer such that it can be operated with the lowest possible maintenance costs.